1. Field
The following description relates to a control channel allocation technology for transmission of control information in a physical layer between a base station and a user equipment (UE), and more particularly, to a control channel managing apparatus of a base station, a control channel searching apparatus of a UE, and a control channel allocation method in a mobile communication system.
2. Description of the Related Art
A base station in a mobile communication system such as 3rd generation long term evolution (3G LTE) of a 3rd generation partnership project (3GPP) transmits data to a user equipment (UE) via a physical downlink shared channel (PDSCH).
Here, a physical downlink control channel (PDCCH) is for transferring downlink control information, such as identifier of the UE, modulation method, and a coding scheme, required for the UE to process data received via the PDSCH, or for transferring uplink wireless resource information (uplink control information) allocated for data to be transmitted from the UE via the uplink channel.
A PDCCH resource is formed with control channel elements (CCEs), and the total number of CCEs of the PDCCH varies in every subframe kε {0, 1, 2, 3, 4, 5, 6, 7, 8, 9}, and the number is represented as NCCE, k.
Since the entire resources of PDCCH should be monitored and decoded in every subframe without prior information, to reduce burden and improve process performance, a search space which is monitored and decoded is designated in every subframe. Each search space has a starting address determined according to a radio network temporary identifier (RNTI).
As shown in FIG. 1, the entire PDCCH is divided into a common space to which control information of data, including system information broadcasting and paging, to be received by all UEs or a plurality of UE groups in a cell is allocated and a UE specific space is allocated to which control information of data is to be transmitted to a particular UE.
The common space is always composed of 16 control channel elements (CCEs), and a base station allocates control information in a UE specific space from a starting address of the search space within a given range.
The number of CCEs required for transmitting one piece of control information is determined according to receiving quality of the PDCCH allocated to a UE, and the number of CCEs is referred to as an aggregation level.
The aggregation level can be one of 1, 2, 4, and 8. FIG. 2 is a table showing the number of CCEs required for transmitting one piece of control information according to the aggregation level, the number of CCEs forming one search space, and the number of pieces of control information allocatable to one search space.
For example, referring to FIG. 2, if the aggregation level is 1, one channel control element is required for transmitting one piece of control information and the number of CCEs forming one search space is 6, so that a total of 6 pieces of control information can be contained in one search space. Therefore, at a UE side, there are six PDCCH control information candidates, and the UE searches for corresponding control information.
If the aggregation level is 4, four CCEs are required for transmitting one piece of control information and one search space is composed of eight CCEs, so that a total of two pieces of control information can be contained in the search space.
A starting address of a search space which contains control information for a UE is allocated is determined by the equation below. Where an aggregation level Lε {1, 2, 4, 8}, a starting address of a UE in a subframe k is represented by Sk(L), and this is defined as follows:
      S    k          (      L      )        =            L      ·              {                              (                                          Y                k                            +              m                        )                    ⁢          mod          ⁢                      ⌊                                          (                                  N                                      CCE                    ,                    k                                                  )                            L                        ⌋                          }              +    i  
Here, i=0, . . . , L−1, m=0, . . . , M(L), and M(L) is the number of PDCCH control information candidates to be monitored in a given search space. Also, Yk is defined as follows:Yk=(A·Yk-1)mod D 
Here, A=39827, D=65537, and Yk-1 denotes the number of RNTI of a UE. Also, the aggregation level is determined as either 4 or 8 for a common space, and Yk is 0.
The total number of CCEs of PDCCH is determined according to the number of transmitter (TX) antennas, the number of physical hybrid automatic request (HARQ) indicator channel (PHICH) groups, the number of physical resource blocks (PRBs), and a value of control format indicator (CFI).
For example, where the number of TX antennas is 4, the number of PHICH groups is 0, the number of PRB is 100 and the value of CFI is 3, the PDCCH is formed of a total of seventy seven CCEs as shown in FIG. 1, the first sixteen CCEs having an address between 0 to 15 are used as a common space, and the remaining sixty one CCEs having an address between 16 to 76 are used as a UE specific space.
FIGS. 3 and 4 are tables showing starting addresses of a search space in a UE specific space for a UE according to RNTIs obtained by the above equations. The table shown in FIG. 3 is obtained in the case where the aggregation level is 8, and the table shown in FIG. 4 is obtained in the case where the aggregation level is 4.
One of a total of 65512 starting from 10 to 65522 may be allocated to the RNTI available for the UE specific space of a UE, and FIGS. 3 and 4 show the starting addresses of the PDCCH search space of each RNTI on the assumption that there are UEs, each having an RNTI between 10 and 19 in a network.
As shown in FIGS. 3 and 4, there may be several RNTIs having the same starting address of a search space when the aggregation level is 4 or 8. In FIG. 3, when the aggregation level is 8 and a subframe is 1 (sf1), the RNTIs 10, 13, 16, and 19 have the same starting address. In FIG. 4, when the aggregation level is 4 and a subframe is 0 (sf0), the RNTIs 10, 12, 14, 16, and 18 have the same starting address.
In these cases, serious problems may occur such as, at maximum, only two RNTIs having the same starting address can be scheduled in a corresponding subframe and the remaining RNTIs cannot operate in the subframe since only two pieces of control information are allocatable to a search space when the aggregation level is 4 or 8.
As such, in the case of a high aggregation level, the more UEs there are having the same starting address in a network, the more network performance is reduced since only two pieces of control information are allocatable in the search space.
For this reason, a technology for enhancing network performance by improving the conventional control channel allocation method is required.